As an apparatus for controlling electric motors that drive an electric train, an electric train control apparatus is available, which has variable-voltage variable-frequency (VVVF) inverters. Also available is an electric train control apparatus that controls a plurality of electric motors of one electric train. In this apparatus, the VVVF inverters control the associated electric motors, respectively.
In the case of an electric train using such a control apparatus as described above, the wheels may slip on a rainy day or a snowy day. While the wheels are slipping, the torque of each electric motor is not conveyed to the rails. Consequently, the travelling ability of the train will decrease.
In this regard, Jpn. Pat. Appln. KOKAI Publication No. 2005-6403, which is a Japanese patent document, discloses an electric train control apparatus that controls the rotation speeds of the electric motor shafts (shaft speeds), achieving a rail adhesion control during the slip, adhering the idling wheels back to the rails during the slip of the wheels.
Available as methods of achieving the rail adhesion control of electric trains are, for example, the acceleration detection control and the slip speed control. When the wheels slip, the shaft speeds of the electric motors abruptly change (increase). In view of this, the shaft speeds of the electric motors are monitored. If the shaft speeds change with time at a rate equal to or higher than a preset value, the acceleration detection control is performed, thereby reducing the torques of the motors.
In the acceleration detection control, the electric train control apparatus determines a reference speed from the shaft speeds of the electric motors. If the shaft speed of any electric motor exceeds the reference speed, the apparatus reduces the torque of the electric motor.
When the electric train control apparatus described above performs the acceleration detection control, it monitors the shaft speed of only one electric motor, thereby achieving the rail adhesion control when the wheels slip. The acceleration detection control had the tendency of reducing the torques of the electric motors too much. Therefore, the average acceleration torque greatly decreases if the wheels slip frequently. As a result, the torque of the electric motor will decrease.
The electric train control apparatus described above monitors a plurality of shaft speeds during the slip speed control, thereby controlling the rail adhesion when the wheels slip. During the slip speed control, the apparatus controls the shaft speeds in accordance with the momentary adhesion force. In this case, the timing of control lags because of the linear delay element. Consequently, the wheels may not be adhered again to the rails. If the wheels on the shafts of all motors, for example, slip, the electric train control apparatus can no longer determine the reference speed. As a result, the rail adhesion cannot be controlled when the wheels slip.
If troubles occur in the lines for transmitting the shaft speeds, in the pulse generators (PGs) for detecting the shaft speeds of the motors or in the inverters, the electric train control apparatus cannot determine the reference speed or may erroneously determine that the wheels slip. In either case, the electric train control apparatus cannot control the rail adhesion.